Just found out he cheated last year

So i was going through my fiancé's email (he asked me if i could look for some car insurance info from last year) and i saw an old email thread dated back in July 2015. It was between him and a girl from his college. He was telling her how my UTIs were getting in the way of our sex life and he was sexually frustrated. At first it just looked like he was venting about our relationship problems at the time, which i had no problem with. I did that too. But then i got flirty and eventually one of her emails said

"I'm so hot. I'm wearing a black corset and red undies, waiting for you"

To which he replied "i'd like to see you in green undies. I'm your master now, i want to see them."

Then after 4 or 5 emails they stopped talking after she said she was going out of town for a weekend and it looks like they never heard from eachother again.

I'm so confused and worried...at the time our relationship was so bad I was very close to leaving him because we were fighting a lot and didn't have really any sex because of my utis that weren't going away with antibiotics. We weren't really too serious back then, hell we never said "i love you" until 6 months after that.

I don't want to leave, it was last year and we were in a horrible relationship before we worked at it around October of 2015. We weren't really in love but it was still a a girlfriend/boyfriend relationship.

Now i'm 20 weeks pregnant, we're engaged and are very happy but i don't know if i should bring up that email i found or of i should let it go. He isn't cheating now, and hasn't since that girl. What angers me the most is that he told her i was his girlfriend and he told her about our problems yet they did things together. She knew he was taken and they just went for it. I'm contemplating sending her an email to know what happened but i also want to let it the fuck go.